Circuit and Method for De-Energizing a Field Coil

ABSTRACT

A circuit includes a first half bridge including a first controllable semiconductor switch and a first diode. The first controllable semiconductor switch is coupled between a first constant supply potential and a center tap of the first half bridge. The first diode is coupled between the center tap and a constant reference potential. A second half bridge includes a second diode and a second controllable semiconductor switch. The second diode is coupled between a second constant potential higher than the first potential and a center tap of the second half bridge. The second controllable semiconductor switch is coupled between the center tap and the constant reference potential. Driver circuitry controls the conducting state of the first and the second semiconductor switch thus controlling the current flow through a field connectable between the center taps.

This is a continuation application of U.S. patent application Ser. No.12/703,361, entitled “Circuit and Method for De-Energizing a FieldCoil,” which was filed on Feb. 10, 2010, and which is herebyincorporated herein by reference in its entirety.

TECHNICAL FIELD

The invention relates to a driver circuit for driving a field coil of agenerator, especially an electric generator (alternator) for automotiveapplications.

BACKGROUND

During the operation of motor vehicle generators sudden surge voltagesoccur in response to a load dump because the magnetomotive force and thestored magnetic energy of the field coil of the generator can only bedegraded at a finite rate. In common generator control circuits thefield coil of a generator is de-energized by means of expensivecomponents, such as power zener diodes, in order to prevent the outputvoltage of the generator to increase to high values which may causedamage to various electric components connected to the generator.

Various circuits for de-energizing field coils in order to reduce themagnetic energy stored therein are known in the art. Although suchcircuits may alleviate the problem of surge voltages that may occurespecially in response to a load dump, there is still the need forimproved circuits that allow a quick and reliable de-energization offield coils.

SUMMARY OF THE INVENTION

A circuit arrangement is disclosed. The circuit arrangement includes afirst half bridge including a first controllable semiconductor switchand a first diode. The first controllable semiconductor switch iscoupled between a first constant supply potential and a center tap ofthe first half bridge. The first diode is coupled between the center tapand a constant reference potential. A second half bridge includes asecond diode and a second controllable semiconductor switch. The seconddiode is coupled between a second constant potential higher than thefirst potential and a center tap of the second half bridge. The secondcontrollable semiconductor switch is coupled between the center tap andthe constant reference potential. Driver circuitry controls theconducting state of the first and the second semiconductor switch thuscontrolling the current flow through a field connectable between thecenter taps.

Further a method for controlling the magnetic energy stored in a fieldcoil of a generator is disclosed. During normal operation, current issupplied to the field coil thus generating a desired magnetic field. Thecurrent flows from a circuit node providing a first constant supplypotential via the field coil to a terminal providing a constantreference potential. A load dump of a load coupled to the generator isdetected. In case a load dump is detected, current supplied to the fieldcoil stopped and a current path is provided from the terminal providingthe constant reference potential via the field coil to a node providinga second constant potential higher than the first supply potential.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be better understood with reference to the followingdrawings and description. The components in the figures are notnecessarily to scale, instead emphasis being placed upon illustratingthe principles of the invention. Moreover, in the figures, likereference numerals designate corresponding parts. In the drawings:

FIG. 1 is a circuit diagram illustrating a known H-bridge forcontrolling the magnetic energy stored in a field coil;

FIG. 2 is a circuit diagram illustrating a first example of the presentinvention wherein for de-energizing the coil a counter voltage isapplied thereto which is higher in magnitude of a system voltage;

FIG. 3 is a circuit diagram illustrating the example of FIG. 2 in moredetail; and

FIG. 4 is a circuit diagram illustrating an alternative to the circuitof FIG. 3.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

As mentioned above, sudden surge voltages may occur during the operationof generators in response to a load dump because the magnetomotive force(i.e., because of the stored magnetic energy of the field coil of thegenerator). For this reason, common generator control units includecircuits for de-energizing the field coil thus considerably reducing theamplitude and the duration of the surge voltages.

FIG. 1 illustrates a so-called H-bridge circuit for reducing the energyof a field coil. The circuit has two power switches M1, M2 which may beimplemented, for example, as MOSFETs or other type of transistors.During normal operation of the circuit, both power switches M₁, M₂ areclosed thus providing current from the system power supply (voltageV_(B)) to the field coil L_(F) of the generator thus generating anexciting field. Thereby a certain amount of magnetic energy is stored inthe coil L_(F). The MOSFETs M1, M2 are usually driven by a pulse-shapedcontrol signal such as, for example, a pulse-width-modulated (PWM)signal, for controlling the average current through the field coil. Aslong as the transistors M₁, M₂ are in a conducting state, current flowsfrom the power system supply (voltage V_(B)) via power switch M1, fieldcoil L_(F), and power switch M2 to ground potential GND.

During a non-conduction state of transistor M₁, when transistor M₁ isdriven in an off-state while transistor M₂ is still on, the current pathV_(B)-M₁-L_(F)-M₂-GND is interrupted and the coil discharges viafree-wheeling diode D1. The effective counter voltage present at thefield coil L_(F) during such a free-wheeling phase is the forwardvoltage of the free-wheeling diode D1, e.g., approximately 0.7 V.

However, in case of a load dump the magnetic energy stored in the coilhas to be reduced more quickly than is achievable in the above-mentionedfree-wheeling phase during normal operation. For a rapid de-energizationof the magnetic energy of field coil L_(F), both power switches M1 andM2 are driven into an off state (non-conduction state). In this case,the current flows via diode D₁, field coil L_(F), and diode D₂. Thereby,current flows through the diodes D₁ and D₂ in a forward direction. Hencethe current flows contrary to the direction of the voltage V_(B) (forexample, 14 V) of the system power supply augmented by the sum of theforward voltages of the diodes D₁ and D₂, i.e., a total of about 15.4 V.The counter-voltage required for this (approximately 15.4 V in thepresent example) is built up by the self-inductance of the field coil.

FIG. 2 is a block diagram illustrating an improved circuit forcontrolling the magnetic energy stored in a field coil L_(F). Similar tothe circuit of FIG. 1, the field coil L_(F) (e.g., for generating anexciting field of a generator) is coupled between the center taps of twohalf bridges thus forming an H-bridge.

A first half bridge is connected between a terminal providing a constantfirst supply potential V_(B) (e.g., from an automotive battery) and aterminal providing a constant reference potential GND. The first halfbridge includes a series circuit of a first controllable semiconductorswitch M₁ and a diode D₁. Thereby the first semiconductor switch M₁ isconnected between the terminal providing the constant first supplypotential V_(B) and the center tap, whereas the first diode D₁ isconnected between the center tap and the terminal providing the constantreference potential GND.

Similarly the second half bridge is connected between a terminalproviding a constant second potential V_(D) higher than the firstpotential V_(B) and the terminal providing the constant referencepotential GND. The second half bridge includes a series circuit of asecond diode D₂ and a second controllable semiconductor switch M₂.Thereby the second Diode D₂ is connected between the terminal providingthe constant second potential V_(D) and the center tap whereas thesecond semiconductor switch M₂ is connected between the center tap andthe terminal providing the constant reference potential GND.

One of ordinary skill will understand that the diodes D1, D2 might bereplaced by other appropriate switching elements which provide the samefunction. Further, the controlled semiconductor switches may beimplemented as, for example, MOSFETs, BJTs, IGBTs or other types oftransistors.

In the example of FIG. 2, the switching state (on or off) of the firstsemiconductor switch M₁ is controlled by a first driver circuitry 10that controls the current flow through the first semiconductor switch M₁and thus the magnetic field provided by the field coil L_(F). Duringnormal operation the strength of the excitation field is controlled bymodulating the driver signal supplied to a control terminal of the firstsemiconductor switch M₁. In many applications this modulation is a pulsewidth modulation. However, other types of modulation (pulse frequencymodulation, pulse density modulation, etc.) may be appropriate. Duringsuch normal operation the second semiconductor switch M₂ is driven intoan on state by a second driver circuitry 20 that is coupled to a controlterminal of the second semiconductor switch M₂ and thus controls thecurrent flow through it. Consequently, during normal operation thecurrent through the field coil L_(F) takes the current pathV_(B)-M₁-L_(F)-M₂-GND or GND-D₁-L_(F)-M₂-GND dependent on the conductionstate of the first semiconductor switch M₁.

In case of a load dump, i.e., in case an electric load suddenlydisconnects from the generator that is excited by the field coil L_(F),both semiconductor switches M₁ and M₂ are driven into an off state.Therefore the second control circuitry 20 is configured to detect suchload dumps, for example, by monitoring the total load current providedby the generator. During this de-energizing mode of operation of thecircuit of FIG. 2 the current through the field coil L_(F) takes thecurrent path GND-D₁-L_(F)-D₂-V_(D), where V_(D) is a constant potentialhigher than the system supply potential V_(B). Thus the resultingcounter voltage present at the field coil is V_(D)+2.V_(F) wherein V_(F)is the forward voltage of the diodes D₁ and D₂. As in the presentexample the field coil L_(F) de-energizes at a higher counter voltage(since constant potential V_(D)>system supply potential V_(B)) comparedto the circuit of FIG. 1 the speed of de-energizing is improved.

The example illustrated in the circuit diagram of FIG. 3 is similar tothe example of FIG. 2. However, the example of FIG. 3 includes onepossibility of how to provide the high potential V_(D). In the presentexample a DC/DC converter 30 is connected between the terminal providingthe constant system supply potential V_(B) and the terminal providingthe constant potential V_(D). As the potential V_(D) is higher thansupply potential V_(B) the DC/DC converter may include a boostconverter.

An alternative to the example of FIG. 3 is illustrated in FIG. 4 bymeans of a further circuit diagram. In this example the boost converter30 is replaced by a zener diode D_(Z), which is connected between theterminal providing the constant system supply potential V_(B) and theterminal providing the constant potential V_(D). Thus, in thede-energizing mode of operation (see description above with reference toFIGS. 1 and 2) the constant potential V_(D) equals the sum of supplypotential V_(B) and zener voltage V_(Z). Thus, during this de-energizingmode of operation of the circuit of FIG. 4 the current through the fieldcoil L_(F) takes the current path GND-D₁-L_(F)-D₂-D_(Z)-V_(B), whereinthe potential present at the common node between diode D₂ and zenerdiode D_(Z) is V_(D)=V_(B)+V_(Z).

Although the present invention has been described in accordance to theembodiments shown in the figures, one of ordinary skill in the art willrecognize there could be variations to these embodiments and thosevariations should be within the spirit and scope of the presentinvention. Accordingly, modifications may be made by one ordinarilyskilled in the art without departing from the spirit and scope of theappended claims.

What is claimed is:
 1. A circuit comprising: a first half bridgeincluding a first controllable semiconductor switch and a firstswitching element, wherein the first controllable semiconductor switchis coupled between a first substantially constant supply potential and acenter tap of the first half bridge, and the first switching element iscoupled between the center tap and a constant reference potential; asecond half bridge including a second switching element and a secondcontrollable semiconductor switch, wherein the second switching elementis coupled between a second substantially constant potential that ishigher than the first substantially constant supply potential and acenter tap of the second half bridge, and the second controllablesemiconductor switch is coupled between the center tap and the secondsubstantially constant potential; and driver circuitry for controlling aconducting state of the first semiconductor switch and the secondsemiconductor switch thus controlling current flow through an inductanceconnectable between the center taps.
 2. The circuit of claim 1, wherein:the first switching element comprises a first diode; and the secondswitching element comprises a second diode.
 3. The circuit of claim 1further comprising: a field coil of a generator coupled between thecenter tap of the first half bridge and the center tap of the secondhalf bridge; wherein the driver circuitry is configured to control anexciting field for the generator by providing a modulated driver signalto the first semiconductor switch during normal operation, and whereinthe driver circuitry is further configured to detect a load dump and todrive the first and second semiconductor switches to an off state incase a load dump has been detected.
 4. The circuit of claim 1, wherein:the center tap of the first half bridge is directly connected to a firstterminal of the first controllable semiconductor switch and to a firstterminal of the first switching element; and the center tap of the firsthalf bridge is directly connected to a first terminal of the secondcontrollable semiconductor switch and to a first terminal of the secondswitching element.
 5. The circuit of claim 1, further comprising aDC/DC-converter coupled to the second half bridge and providing thesecond constant potential.
 6. The circuit of claim 5, wherein theDC/DC-converter is coupled between the second constant potential and thefirst constant supply potential.
 7. The circuit of claim 1, furthercomprising a zener diode coupled between a terminal providing the firstconstant supply potential and a terminal providing the second constantpotential.
 8. The circuit of claim 7, wherein the second constantpotential is the sum of the first constant supply potential and a zenervoltage of the zener diode.
 9. A circuit comprising: a first switchcoupled between a first supply voltage node and a first terminal of aninductance; a first diode coupled between the first terminal of theinductance and a reference voltage node; a second diode coupled betweena second supply voltage node and a second terminal of the inductance,wherein the second supply voltage node is configured to carry a highervoltage than the first supply voltage node; and a second switch coupledbetween the second terminal of the inductance and the reference voltagenode.
 10. The circuit of claim 9, further comprising a DC/DC convertercoupled between the first voltage node and the second voltage node. 11.The circuit of claim 9, further comprising a Zener diode coupled betweenthe first voltage node and the second voltage node.
 12. The circuit ofclaim 9, wherein the first supply voltage node comprises a terminal ofan automotive battery.
 13. The circuit of claim 9, further comprisingdrive circuitry coupled to a control terminal of the first switch and acontrol terminal of the second switch.
 14. The circuit of claim 13,wherein the drive circuitry is configured to provide a modulated driversignal to the first switch during normal operation.
 15. The circuit ofclaim 14, wherein the drive circuitry is further configured to turn offthe first switch and the second switch when a load dump has beendetected.
 16. The circuit of claim 9, further comprising a third diodecoupled in parallel with the first switch and a fourth diode coupled inparallel with the second switch.
 17. The circuit of claim 9, wherein thefirst supply voltage node is coupled to a 14 volt supply and wherein thereference voltage node is a ground node.
 18. A circuit comprising: aload dump detector coupled to a load of a generator; and a first circuitconfigured to: supply current to a field coil of a generator from acircuit node providing a first substantially constant supply potentialvia the field coil to a terminal providing a substantially constantreference potential during a normal operation, and stop supplyingcurrent to the field coil and providing a current path from the terminalproviding the constant reference potential via the field coil to a nodeproviding a second constant potential that is higher than the firstconstant supply potential when a load dump is detected by the load dumpdetector.
 19. The circuit of claim 18, further comprising a DC/DCconverter configured to provide the second constant potential.
 20. Thecircuit of claim 18, wherein the second constant potential is providedby a zener diode coupled between a node providing the first constantsupply potential and a node providing the second constant potential, thesecond constant potential being the sum of the first constant supplypotential and a zener voltage of the zener diode.
 21. A method ofoperating a circuit having a first switch coupled between a first supplyvoltage node and a first terminal of an inductance, a first diodecoupled between the first terminal of the inductance and a referencevoltage node, a second diode coupled between a second supply voltagenode and a second terminal of the inductance, and a second switchcoupled between the second terminal of the inductance and the referencevoltage node wherein the second supply voltage node is configured tocarry a higher voltage than the first supply voltage node; and providinga first supply voltage to the first supply voltage node; and providing asecond supply voltage to second supply voltage node, wherein the secondsupply voltage is higher than the first supply voltage.
 22. The methodof claim 21, further comprising: providing a modulated drive signal to afirst switch during normal operation; detecting a load dump condition;and turning off the first switch and the second switch when the loaddump condition has been detected.
 23. The method of claim 21, whereinproviding the second supply voltage comprises coupling a DC/DC converterbetween the first supply voltage node and the second supply voltagenode.
 24. The method of claim 21, wherein providing the second supplyvoltage comprises coupling a Zener diode between the first supplyvoltage node and the second supply voltage node.
 25. The method of claim21, wherein providing the first supply voltage comprises coupling anautomotive battery to the first supply voltage node.